The alignment of Earth and Venus dictated that
Magellan be launched between April 28 and May 29, 1989, with the best
launch occurring on May 5. (The advantage of a May 5 launch date would
become apparent during the VOI maneuver, when the fixed performance of
the SRM would be the exact value required to put Magellan into the desired
orbit.) The Mission Design Team had skillfully maximized the number of
possible launch days so that delays caused by weather or shuttle problems
would not prevent a launch. The next few paragraphs attest to the wisdom
of this planning effort.

The Space Shuttle Atlantis was moved to Launch
Pad 39B at Cape Canaveral, Florida, on March 22, 1989, and the Magellan/IUS
combination was installed in the cargo bay on March 25.

In preparation
for launch activities, Magellan team members from JPL who were part of
the launch team assumed their command posts at the Kennedy and Johnson
Space Centers. During the preceding months, they and representatives
from these other two NASA centers underwent extensive training for the
launch event. Other team members remained at JPL to monitor the spacecraft's
state of readiness as it rested inside the shuttle's cargo bay. Magellan
team members across the country shared the excitement as the launch status
remained green (for "go"). Five people who couldn't be more ready were
the STS-30 Atlantis astronauts: Captain David M. Walker, Commander; Colonel
Ronald J. Grabe, Pilot; and Mission Specialists Mary L. Cleave, Ph.D.,
Major Mark C. Lee, and Norman E. Thagard, M.D.

The countdown began April
24 and proceeded smoothly toward an April 28 launch. On the 28th, however,
with the countdown at Launch -31 seconds, the automatic ground software
system detected a shuttle problem and the countdown came to a halt. A
hydrogen recirculation pump that cooled the shuttle engines prior to firing
developed a short and stopped. The launch on this day was scrubbed.

After careful review of the pump problem and of a second problem involving
abnormal venting of a hydrogen circulation line, the launch team selected
May 4 for the next attempt.

The countdown resumed, starting this time
at Launch -2 days, and proceeded smoothly. But May 4 did not dawn as
a likely day for a launch. The sky was overcast, and strong crosswinds
(greater than 12 knots) blew across the runway at the Kennedy Space Center's
emergency landing site. No one was surprised when a hold for weather
was called at Launch -5 minutes.

Fortunately, a 64-minute launch window
had been designed for May 4. After 59 anxiety-filled minutes, the winds
dissipated and the clouds parted just enough for launch at 2:46:59 p.m.,
eastern daylight time (see Figure 9-2), only 5 minutes before the end
of the launch window for that day. The shuttle slowly rose out of the
billows of steam and accelerated toward the low clouds. It went briefly
out of sight and then reappeared for a few seconds, framed in a blue window
amid the clouds. It was truly picture perfect.

The Space Shuttle Atlantis
compensated for the delay in launch by yaw steering into the correct orbit
plane. After five revolutions around the Earth at an altitude of 296
kilometers (160 nautical miles), Magellan was slowly deployed from the
shuttle (see Figure 9-3). Sixty minutes later, with the solar panels
extended as shown in Figure 9-4, the IUS ignited its two SRMs in rapid
succession and propelled the spacecraft on very nearly the precise trajectory
to Venus. After firing its attitude-control thrusters for a small course
correction, the IUS separated from Magellan and used its remaining fuel
to move away from the spacecraft.

Figure 9-3. After five revolutions around the Earth, Magellan and its
IUS booster were deployed from the Shuttle.

Figure 9-4. Magellan's solar panels were extended prior to IUS
ignition because the booster's roll-control thrusters were too close to the
ends of the panels in their stowed position.

Figures 9-3 and 9-4 are two photographic
mementos the astronauts brought back to the Magellan team.

The original May 1988 launch period would have allowed Magellan
to reach Venus 4 months later via a Type-I trajectory, meaning that from
launch to destination, the spacecraft would have traveled less than 180 degrees
around the Sun. There was a similar opportunity in the October 1989 launch
period initially set aside for Magellan but sub-sequently assigned to
the Galileo mission to avoid further delays in its launch.

However, the
positions of Earth and Venus during the late-April to late-May 1989 launch
period required a Type-IV trajectory (see Figure 9-5). This meant that
the spacecraft would travel between 1-1/2 to 2 times around the Sun (slightly
more than 540 degrees) and that it would arrive at Venus on August 10,
1990. While it dictated a longer cruise duration (15 months), the Type
IV actually had the advantages of reductions in launch energy and Venus
approach speed.

Since launch, Magellan has traveled more than 1-1/2 times
around the Sun at an average speed of 113,600 kilometers per hour (71,000
miles per hour) relative to the Sun and has logged over 1.261 billion
kilometers (788 million miles). Three trajectory-correction maneuvers
(TCMs) have kept the spacecraft on track for the correct aim point and
arrival time at Venus. The TCMs were executed on May 21, 1989, and on
March 13 and July 25, 1990.

Magellan's Type-IV
trajectory and the resultant Venus arrival date brought about some changes
in the basic mapping plan developed for the 1988 mission.

Superior conjunction
(where the Sun is positioned between Venus and the Earth) will now occur
during the primary mapping mission, instead of at the end. The result
is that up to 18 days of mapping data will be lost around November 2,
1990, because radio interference from the Sun will make it impossible
to communicate with the spacecraft. Fortunately, the missing data can
be recovered in early July 1991, if the mission is extended for additional
243-day mapping cycles.

The trajectory also dictates an approach over
the north pole; this will result in a mapping swath from north to south,
the reverse of that planned for the 1988 mission.

The
word "cruise" conjures images of leisure, spare time, and relaxation.
It is true that people who work on interplanetary missions usually take
some time after launch to reflect on what it has taken to get that far
and on what lies ahead to ensure a successful mission. But it's the "what
lies ahead" that makes this period of reflection indeed brief.

Magellan
team members have been occupied with two primary tasks during the cruise
to Venus. The first was to fly the spacecraft and evaluate the performance
of its various subsystems and components in the actual space environment.
Ground test chambers are the next best thing to being there, but they
cannot completely simulate interplanetary conditions. The second task
was to plan and prepare for the activities that will occur in Venus orbit.

The cruise period has not been
a time of leisure for the spacecraft either.

Magellan has traveled farther
from the Sun than Earth's orbit (149,669,000 kilometers or 93,000,000
miles) and has approached to within 104,640,000 kilometers (65,400,000
miles) of the Sun, 2,880,000 kilometers (1,800,000 miles) closer to the
Sun than the orbit of Venus. This changing environment allowed us to
characterize the thermal responses of various parts of the spacecraft
over a range of temperatures as these parts faced toward or away from
the Sun. Knowing these responses is referred to by spacecraft engineers
as "having a model." The ability to refine and validate the thermal model
means that we will be better able to predict the thermal response of the
spacecraft once it is in Venus orbit.

Similarly, the spacecraft power
models, both for input from the solar arrays and output from the batteries,
were validated as we performed cruise activities that required varying
power output.

A series of "guide-star" calibrations was carried out
during cruise to determine precisely how the star scanner responds to
the set of stars we plan to use for accurate spacecraft pointing during
the prime mapping mission. These calibrations are called STARCALs.

Magellan
is a three-axis-stabilized craft that relies on three reaction wheels
to provide attitude (pointing) control (see Chapter 4). Four gyroscopes
provide the information required to determine the attitude. Because extremely
high pointing accuracy is required to successfully capture radar reflections
from the planet's surface, several calibrations were conducted on the
gyroscopes to provide a thorough understanding of their orientation and
behavior.

Two types of gyroscope calibrations were conducted to correct
two possible error sources. The Scale Factor Calibration (SFCAL) allows
correction of the difference between the amount the spacecraft thinks
it has turned and the amount it has actually turned in a large-angle excursion.
The Attitude Reference Unit Calibration (ARUCAL) allows correction of
the offset of the axes of the gyroscope assembly relative to the star
scanner reference frame. During flight, this offset can change from the
amount measured before launch.

Pointing of the HGA was calibrated to
assure its accuracy while performing the dual functions of radar mapping
and telecommunications. This activity is called an HGACAL.

Another high-precision
task was determining the desired orbit and its timing. Useful SAR images
can be obtained only if the exact range from the spacecraft to the planet's
surface is known throughout each mapping pass. Because Magellan's orbit
will be highly elliptical, the range to the surface will change every
moment and require frequent adjustments to the radar commands. Accurate
calculation of the needed adjustments is totally dependent on precise
knowledge of the orbit.

The orbit-determination task relies on a navigation
technique called "differenced Doppler," which involves measurements of
the spacecraft's signal using tracking antennas at the Spain and California
(and sometimes the Australia and California) Deep Space Network (DSN)
complexes. Obtaining these measurements during cruise refined the techniques,
verified the procedures to be used in orbit, and assured us that the differenced
Doppler approach will provide sufficient orbit-prediction accuracy to
guarantee good radar-data collection.

Other major ground test activities
that involved interaction with the spacecraft were the Mapping Readiness
Tests carried out at the DSN sites; these tests verified that the DSN
is primed to support mapping operations. Magellan will send 1.8 gigabits
of data back to Earth during every orbit. Because the data will be stored
on tape recorders during each mapping pass and overwritten with new data
during the next pass, there will be only one chance during each orbit
to send the data to a DSN station. Additionally, the timeline allows the
station only one minute to lock on the spacecraft's signal before the
data flow begins. The DSN's lockup and recording operations must occur
without a hitch to avoid gaps in the Magellan Venus map. Results of the
Mapping Readiness Tests verified that lockup can occur within one minute
and validated the operational procedures for capturing all of the data
from the spacecraft.

In December 1989, the radar electronics were turned
on for the first time since before launch. Both the radar system and
the hardware passed muster. This test paved the way for a more complicated
test performed in May 1990, when the radar and the spacecraft were put
through their paces for more than three days. The spacecraft turned through
the intricate series of maneuvers it will perform orbit after orbit as
it maps the planet. At the same time, the radar system issued its complex
series of mapping commands. This period of simulated mapping operations
allowed us to verify many spacecraft and ground procedures and much of
the mapping software that will drive Magellan once it is in orbit around
Venus.

Magellan has also performed some routine "housekeeping" activities.
Star scans were performed daily to allow correction for the normal drift
in spacecraft pointing, and the reaction wheels were desaturated twice
daily to eliminate the momentum accumulated from small torques to the
spacecraft caused by the Sun's radiation. These two activities are discussed
in more detail in Chapter 4.

Familiarity
with the spacecraft's in-flight characteristics gained during the first
few months following launch allowed us to take a critical look at our
plans for both the in-orbit checkout (IOC) and mapping phases and revise
them where needed.

In-orbit checkout, a thorough examination of the spacecraft
and the radar, will be the first event after achieving Venus orbit. Final
planning for this activity took almost a year. Assembling the requirements
for in-orbit tests, resolving conflicts between requirements, negotiating
a fundamental plan, and working out the operational and procedural details
was an intense effort conducted in parallel with the activities involved
in flying the spacecraft.

Final planning for the primary mapping mission
was also achieved during this period. The prime mission involves three
distinct types of geometry: nonocculted mapping, the superior conjunction
phase, and the apoapsis occultation phase. Each type places different
constraints on the mapping plan, and each was analyzed and updated separately.

The
results of the planning efforts for IOC and mapping are described in Chapters
10 and 11, respectively.

Placing a spacecraft into
a precise orbit around a planet millions of miles away, checking out its
equipment and subsystems to make sure they are working properly, and pronouncing
the spacecraft ready for mapping operations are responsibilities and pressures
definitely a cut above those we face on a daily basis. But the Magellan
team will perform this scenario throughout the VOI maneuver and the IOC
phase. As with any well-orchestrated production, an intensive period
of rehearsal has been essential.

The eight-member Mission Engineering
Team has devoted a portion of the cruise period to developing and conducting
operational readiness tests and various training exercises, including
simulated anomalies, that have tested and evaluated our performance in
carrying out the major mission functions mentioned above. These dress
rehearsals allowed us to refine our procedures and techniques as we went
through the actual processes and interfaces (some of which are complex
and time critical) that will be required. Now we feel we are ready for
the real thing.